A common secondary recovery method for increasing the production rate of a petroleum reservoir and the recoverable petroleum reserves is to inject water into the reservoir. The injection water is conducted into the reservoir at a pressure that drives out further hydrocarbons therefrom.
Due to various reasons, the injection water normally must be treated with chemicals prior to being injected into the reservoir. Water for injection is generally taken from the closest available water source, for example saline seawater, and normally there will be various organic and inorganic materials present in the water. The organic material comprises, among others, zooplankton, phytoplankton, bacteria and similar.
Organic material in the injection water may bring about unfavorable conditions and results downstream of the inlet point of the water. This may occur in the form of biofouling, sedimentation and/or scaling taking place on equipment located upstream of the well, within the well and downstream thereof. For example, such equipment may include sulphate treatment equipment for removing sulphates (SO4) from the injection water.
Such organic material in the injection water may also carry, among others, anaerobic, sulphate-reducing bacteria. When, for example, seawater is injected into a hydrocarbon-containing reservoir, substantial amounts of said sulphates along with carbon compounds are introduced into the reservoir. Thus, the anaerobic, sulphate-reducing bacteria are given excellent conditions for generating hydrogen sulphide (H2S), which may cause reservoir souring, among other things. Hydrogen sulphide gas is also very corrosive and may have a very destructive effect on well-related equipment. The gas is also toxic and may be injurious to health and may be directly life-threatening if, in this connection, coming into contact with living creatures.
Oftentimes the efficient destruction of organic material, particularly bacteria, in the injection water is therefore a prerequisite for allowing water injection to take place in a subsurface reservoir. The water treatment would be even more efficient if also the nutritive organisms and substances for the bacteria were destructed in so doing. After the treatment, the water would thus lack nutrients for any surviving bacteria.
Failure in the destruction of organic material, however, may result in reservoir souring, corrosion problems and gradual build-up of bacteria slime and various deposits and similar in the reservoir and in the well.